JP2000042621A - Cooling control method of hot rolled steel plate - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce temperature nonuniformity of a steel plate by dividing cooling control into forward stage cooling and backward stage cooling, executing forward stage cooling with overall film boiling cooling and successively executing backward stage cooling with overall nucleate boiling cooling up to a prescribed cooling stopping temperature. SOLUTION: A steel plate 1 of high temperature is transported to a forward stage cooking zone and a backward stage cooling zone with a table roller 2. The upper surface of the steel plate 1 is cooled with cooling water from slit nozzles 4 and the rear surface of the same is cooled with cooling water from spray nozzles 5 in the forward stage cooling zone and the backward stage cooling zone. Cooling water is drained by draining roll 6. Water quantity density of cooling water in forward stage cooling is specified to 100-300 l/min.m3 and a surface temperature of the steel plate 1 when forward stage cooling is finished is specified to 550-600 deg.C. Water quantity density when backward stage cooling is started is specified to >=800 l/min.m3. The length of the forward stage cooling zone and the backward stage cooling zone is suitably changed based on a plate thickness, a cooling speed and a cooling stopping temperature or the like.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for controlling and cooling a hot-rolled high-temperature steel sheet, particularly a thick steel sheet.

[0002]

2. Description of the Related Art In recent years, as a process for manufacturing a thick steel plate, the application of an online controlled cooling method for controlling and cooling a hot steel plate immediately after rolling online has been expanding. According to such an on-line controlled cooling method, high strength and high toughness can be imparted to a steel sheet, alloy elements contained in the steel can be reduced, and heat treatment can be reduced. For example, a cost reduction effect can be obtained.

However, in general, a hot-rolled high-temperature steel sheet is not necessarily uniform in temperature distribution, sheet shape, surface properties, and the like, so that uneven cooling, that is, temperature unevenness is easily generated in the steel sheet during cooling. Deformation on steel plate after cooling,
There is a problem that residual stress, material non-uniformity, and the like occur, resulting in poor product quality and operational trouble.

In view of the above, conventionally, there have been proposed many means for uniformly cooling a high-temperature steel sheet to suppress the occurrence of temperature unevenness. For example, the following techniques have been disclosed. (1) Japanese Patent Application Laid-Open No. 62-289316: cooling of a steel sheet is divided into two stages of pre-stage cooling and post-stage cooling, and rapid cooling is performed so that the surface temperature of the steel plate is reduced by 100 ° C. or more by pre-stage cooling.
Next, by cooling the steel sheet to a predetermined temperature by post-stage cooling, the temperature difference in the width direction of the steel sheet is reduced (hereinafter referred to as Prior Art 1).

(2) JP-A-7-284836: Cooling of a steel sheet is divided into two stages of pre-stage cooling and post-stage cooling, and the cooling is temporarily stopped between the pre-stage cooling and the post-stage cooling to recover the steel plate surface temperature. The pre-stage cooling is adjusted so as to be heated to 650 to 750 ° C., and in the post-stage cooling, cooling is performed with a desired amount of cooling water to improve the uniformity of the steel sheet temperature (hereinafter, referred to as
Prior art 2).

FIG. 2 is a schematic side view of an apparatus for carrying out the method of the prior art 2, which uses a cooling device comprising a primary cooling zone a, a recuperating zone b, and a secondary cooling zone c, and uses a table roller. After the steel sheet 1 transferred by the cooling device 2 is cooled by the cooling water injected from the cooling nozzle 8 in the primary cooling zone a, the steel sheet 1 is reheated in the recuperation zone b. At 650 ° C., the transformation of the steel sheet is prevented from occurring as much as possible. Then, in the secondary cooling zone c, the steel sheet is cooled by cooling water injected from the cooling nozzle 9 to shorten the film boiling duration, and the nucleate boiling is promptly started. Has been migrated. 10 is an air nozzle.

(3) JP-A-57-152430:
In order to suppress the surface hardening of the steel sheet, it is a method of slowly cooling the steel sheet in the first stage and strongly cooling it in the second stage.
Cool at a water mass density of 0.7 m ³ /min.m ² , and in a temperature range below that, the water mass density of 1.0 m ³ /min.m ² is maintained until the steel plate center temperature becomes 500 to 450 ° C or less. (Hereinafter referred to as prior art 3).

[0008]

However, in the method of the prior art 1, when uneven cooling or deformation occurs in the steel sheet in the pre-stage cooling, in the subsequent post-stage cooling,
As a result of the accumulation of the uneven cooling and deformation during the pre-stage cooling, there arises a problem that the temperature difference in the steel sheet further increases.

According to the method of the prior art 2, since a uniform nucleate boiling state is obtained over the entire surface during the post-stage cooling, the uniformity of the temperature distribution of the steel sheet is improved. However, since the surface of the steel sheet is rapidly cooled by nucleate boiling, the hardness of the steel sheet surface after cooling increases, and an undesirable hardness distribution occurs in the thickness direction of the steel sheet. In particular, since the transformation has not progressed in the pre-stage cooling, the surface layer portion of the steel sheet changes from austenitic structure to bainite or martensite structure by rapid cooling, so that a remarkable increase in surface hardness cannot be avoided.

According to the method of suppressing surface hardening of the prior art 3, since the pre-cooling is the cooling of the transition boiling region, nucleate boiling is locally generated, and the temperature unevenness between the portion and the film boiling is caused. That was inevitable to happen. In addition, since the temperature of the steel sheet rapidly decreases in the portion where nucleate boiling occurs in the pre-stage cooling, even if the average temperature of the entire steel sheet surface falls within a desired temperature range, the surface temperature of the portion where nucleate boiling occurs is not reduced. It was inevitable that the temperature dropped below the average temperature and the steel sheet surface in that portion hardened.

Accordingly, an object of the present invention is to solve the above-mentioned problems, and to reduce the occurrence of temperature unevenness during cooling of a hot-rolled high-temperature steel sheet in a method for controlling and cooling a hot-rolled steel sheet, An object of the present invention is to provide a cooling method capable of obtaining a steel sheet having good flatness and a small difference in hardness in the thickness direction over the entire surface.

[0012]

In general, when a high-temperature steel sheet is water-cooled, first, as shown in FIG. 3, a film boiling state occurs in which a vapor film exists between the steel sheet surface and the cooling water. As the surface temperature of the steel sheet decreases, the film transitions from film boiling to transition boiling, and when the surface temperature of the steel sheet further decreases, almost the entire surface of the steel sheet comes into contact with the cooling water, and a state in which steam bubbles are locally foamed, that is, nuclei. Boiling, the steel sheet reaches a low temperature through such film boiling, transition boiling and nucleate boiling.

When the surface temperature of the steel sheet is in the transition boiling region, the cooling portion is delayed at the start of cooling due to the lower heat flux of the high temperature portion compared with the low temperature portion, whereas the low temperature portion is Conversely, the cooling is promoted because the heat flux is large. As a result, the temperature difference between the high-temperature portion and the low-temperature portion at the start of cooling increases. As long as cooling is performed in the transition boiling region, local temperature unevenness is accumulated and expanded, and the flatness of the steel sheet after cooling is poor,
In addition to the residual stress, unevenness such as hardness distribution and strength distribution, that is, unevenness of material occurs.

On the other hand, in the film boiling region or the nucleate boiling region, a portion having a high temperature at the start of cooling has a higher heat flux than a portion having a low temperature, and thus cooling is promoted. On the other hand, in the portion where the temperature is low, the cooling is delayed due to the low heat flow rate. As a result, the temperature difference between the two is reduced, and the temperature unevenness is reduced. Therefore, if the entire cooling process is performed only by film boiling or only nucleate boiling, uniform cooling without temperature unevenness can be achieved.

However, in the case of film boiling cooling, the cooling capacity is low. Therefore, if an attempt is made to cool to a desired cooling stop temperature only by film boiling, a cooling rate required for material control cannot be obtained.

On the other hand, in nucleate boiling cooling, since the cooling capacity is too high, if an attempt is made to cool to a desired stop temperature only by nucleate boiling, the surface temperature of the steel sheet drops to 100 ° C. during cooling, and bainite or martensite To occur,
Surface hardening is inevitable.

The inventors of the present invention have carefully studied the cooling conditions of a steel sheet that suppresses surface hardening and does not cause temperature and material unevenness. As a result, the cooling is divided into two stages. Cooling while suppressing the occurrence of temperature unevenness by surface film boiling cooling, promotes ferrite transformation of the surface layer, raises the ferrite fraction in advance, and in the latter stage cooling, uniformly cools the whole surface by nuclear boiling cooling and cools to the predetermined cooling stop temperature Then, it was found that temperature unevenness and material unevenness did not occur.

The present invention has been made on the basis of the above findings. According to a first aspect of the present invention, there is provided a method for on-line controlled cooling of a hot-rolled high-temperature steel sheet, wherein the controlled cooling is performed in a first stage. Divided into two stages of cooling and post-stage cooling, wherein the pre-stage cooling is performed by whole-surface film boiling cooling, and the subsequent post-stage cooling is performed by full-surface nucleate boiling cooling to a predetermined cooling stop temperature. It is.

According to a second aspect of the present invention, the water density of the cooling water in the pre-stage cooling is set in the range of 100 to 300 l / min.m ² , and the surface temperature of the steel sheet at the end of the pre-stage cooling is 550.
To 600 ° C., and the water density of the cooling water at the start of the post-stage cooling is set to 800 l / min.m ² or more.

[0020]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS In the present invention, as described above, the pre-stage cooling is performed by the whole-surface film boiling cooling.
Promotes ferrite transformation of the surface layer while suppressing the occurrence of temperature unevenness. As a result of a detailed study of the end temperature of such pre-stage cooling, the steel plate surface temperature was 550 ° C.
It has been found that 0 ° C. or less is desirable. If the end temperature of the pre-stage cooling is lower than 550 ° C., bainite transformation starts in the surface layer of the steel sheet, and the surface layer may be hardened. On the other hand, when the end temperature of the pre-cooling exceeds 600 ° C., the ferrite transformation of the surface layer becomes insufficient.

Further, as a result of a detailed study of the water density of the cooling water in the pre-stage cooling, it was found that the water density in the pre-stage cooling was desirably in the range of 100 to 300 l / min.m ² . Water density during pre-stage cooling is 1
If it is less than 00 l / min.m ² , the cooling rate will be low, the cooling time in the first stage will be long, and the productivity may decrease. on the other hand,
If the water density exceeds 300 l / min.m ² , it becomes difficult to maintain film boiling over the entire surface, and nucleate boiling occurs partially,
There is a problem that causes uneven temperature and uneven surface hardness.

A close examination of the water density of the cooling water at the start of the post-stage cooling revealed that the water density required to maintain a stable nucleate boiling state over the entire surface of the steel sheet varies greatly depending on the steel sheet surface temperature. Steel sheet surface temperature when cooling steel sheets of various temperatures with slit nozzle,
FIG. 4 shows the result of examining the relationship between the cooling water required to maintain nucleate boiling stably over the entire surface of the steel sheet and the water density.
FIG. 4 shows that the nucleate boiling can be stably maintained by setting the steel sheet surface temperature to 600 ° C. or less and the water density at the start of the post-stage cooling to 800 l / min.m ² or more.

If the cooling system is a system in which the pre-stage cooling is in a state of film boiling and the post-stage cooling is in a state of nucleate boiling,
Although not particularly limited, it is preferable to perform spray cooling or cooling with a slit nozzle from the viewpoint of cooling uniformity.

[0024]

Next, the present invention will be described with reference to embodiments. FIG. 1 is a schematic side view showing an example of an apparatus for performing the method of the present invention. As shown in FIG. 1, a hot-rolled hot steel sheet 1 is transferred to a control cooling device by a table roller 2 provided at a constant pitch. The control cooling device includes a table roller 2 above the table roller 2.
A plurality of draining rolls 6 are provided in pairs, and a slit nozzle 4 from the upstream roll to the downstream roll is provided on the upper surface side between the draining rolls 6, and a spray nozzle is provided on the lower surface side. A nozzle 5 is provided. Reference numeral 3 denotes a cooling water supply header, and reference numeral 7 denotes a flow control valve.

The total length of such a control cooling device is, for example, about 20 m, and is composed of 20 table roll units in which a front cooling zone and a rear cooling zone are combined. The combination of the former cooling zone and the latter cooling zone depends on the sheet thickness, cooling rate, cooling stop temperature, etc.
You can choose freely.

While the steel sheet 1 is conveyed by the table roller 2 through the pre-stage cooling zone and the post-stage cooling zone,
The steel plate upper surface is cooled by a slit nozzle method, and the steel plate lower surface is cooled by a spray method. The water flow density of the cooling water in each zone is adjusted by the flow control valve 7 from a small water flow to a large water flow.

C: 0.12 wt.%, Si: 0.3 wt.%, M
n: A steel slab having a chemical composition containing 1.3 wt.% is heated in a heating furnace to a temperature of 1150 ° C., and is then subjected to hot rolling at a thickness of 30 mm, a width of 3000 mm and a length of 1200 mm.
It was hot rolled into a 0 mm steel plate.

The hot-rolled high-temperature steel sheet was transferred to the control cooling device shown in FIG. 1 by the table roller 2, cooled in the first cooling zone, and subsequently cooled to the target cooling stop temperature in the second cooling zone. The cooling rate and cooling stop temperature were controlled by the amount of water in the pre-stage cooling device, the number of zones used, the transfer speed of the steel sheet, and the like. The steel sheet surface temperature at the end of the pre-cooling was determined by heat transfer calculation, and the temperature distribution of the steel sheet after the final cooling was measured by a scanning radiant cooling thermometer.

Table 1 shows that the number of pre-cooling zones, the number of post-cooling zones, the conveying speed of the steel sheet, and the number of pre-cooling zones when the hot-rolled hot steel sheet was cooled from about 800 ° C. to about 500 ° C. under various conditions as described above. Pre-cooling water volume density, post-cooling start temperature,
The following shows the cooling water mass density, cooling stop temperature, temperature distribution of the steel sheet after cooling, hardness difference in the thickness direction of the steel sheet, and hardness distribution in the steel sheet plane. In Table 1, Nos. 1-4 are examples of the present invention,
o. 5 to 10 are comparative examples.

[0030]

[Table 1]

In Table 1, the temperature distribution of the steel sheet after cooling is based on the difference between the maximum temperature and the minimum temperature in the plane of the steel sheet.
The following was evaluated. The lower the temperature difference, the less the temperature unevenness, indicating that the temperature uniformity of the steel sheet is better.

○: When the difference between the maximum temperature and the minimum temperature in the steel sheet is within 20 ° C. Δ: The difference between the maximum temperature and the minimum temperature in the steel sheet exceeds 20 ° C.
In the case of 50 ° C ×: The difference between the maximum temperature and the minimum temperature in the plane of the steel sheet is more than 50 ° C. The hardness of the steel sheet is evaluated by Vickers hardness under a load of 10 kg, and the thickness between the steel sheet surface and the center of the sheet thickness. Based on the hardness difference in the direction and the hardness distribution in the steel sheet surface, the following evaluation was made. (1) Hardness difference in the thickness direction: ：: When the difference between the surface hardness and the hardness at the center of the thickness is within 20 in Hv Δ: The difference between the surface hardness and the hardness in the center of the thickness is more than 20 in Hv
In the case of 40: ×: The difference between the surface hardness and the center thickness of the sheet thickness is more than 40 in Hv. (2) In-plane hardness distribution of the steel sheet: ○: The difference between the maximum and the minimum in the surface hardness of the sheet is H.
v: within 20 Δ: The difference between the maximum and minimum surface hardness of the steel sheet is H
v: more than 20 to 40: ×: The difference between the maximum and the minimum in the steel sheet surface of the surface hardness is H
As shown in Table 1, when the pre-stage cooling water volume density was high at 500 l / min.m ² and the pre-stage cooling could not be performed by full-surface film boiling cooling as shown in Table 1, The uniformity of the temperature distribution of the steel sheet after cooling was poor, and the hardness difference in the thickness direction was large. In particular, the hardness distribution in the steel sheet plane was extremely poor. As in Comparative Example 6, when the post-stage cooling water volume density was low at 600 l / min.m ² and the post-stage cooling could not be performed by nucleate boiling cooling, the uniformity of the temperature distribution of the steel sheet after cooling was low. It was extremely poor, and the hardness distribution in the steel sheet was also poor.

As in Comparative Example 7, when the steel plate surface temperature at the end of the pre-stage cooling, ie, at the start of the post-stage cooling, was high at 700 ° C., and the pre-stage cooling could not be performed by the film boiling cooling of the entire surface,
The uniformity of the temperature distribution and the hardness distribution in the plane of the steel sheet after cooling were poor, and the difference in hardness in the thickness direction was particularly large. As in Comparative Example 8, when the steel sheet surface temperature at the end of the pre-stage cooling, ie, at the start of the post-stage cooling, was low at 510 ° C. and the pre-stage cooling could not be performed by the film boiling cooling on the entire surface, the temperature distribution of the steel sheet after cooling , The hardness difference in the thickness direction, and the hardness distribution in the plane of the steel sheet were poor.

As in Comparative Example 9, when post-stage cooling consisting of nucleate boiling cooling was not performed, the difference in hardness in the plate thickness direction was extremely large. Then, as in Comparative Example 10, air cooling was performed between the pre-stage cooling and the post-stage cooling, and the water volume density of the pre-stage cooling was extremely high at 2000 l / min.m ² , and at the end of the pre-stage cooling, ie, at the start of the post-stage cooling. Steel plate surface temperature is 69
When the temperature is high at 0 ° C. and the pre-stage cooling cannot be performed by the film boiling cooling on the entire surface, the temperature distribution of the steel sheet after cooling and the hardness distribution within the steel sheet plane are poor. It was great.

On the other hand, in the case of Examples Nos. 1 to 4 of the present invention in which the pre-stage cooling was performed by the whole-surface film boiling cooling and the subsequent post-stage cooling was performed by the whole-surface nuclear boiling cooling, the temperature distribution of the steel sheet after cooling was low. The hardness was uniform, the difference in hardness in the thickness direction and the difference in hardness in the plane of the steel plate were small, and the uniformity of the material was excellent.

[0036]

As described above, according to the method of the present invention, when controlling and cooling a hot-rolled steel sheet, the occurrence of temperature unevenness during cooling is reduced, and the flatness after cooling is improved. In addition, a steel sheet having a small hardness difference in the thickness direction over the entire surface can be obtained, which eliminates the need for re-correction and maintenance of the steel sheet after cooling, reduces the variation in material, and improves the production yield. And other industrially useful effects.

[Brief description of the drawings]

FIG. 1 is a schematic side view showing an example of an apparatus for performing the method of the present invention.

FIG. 2 is a schematic side view showing an example of a conventional cooling device.

FIG. 3 is a diagram showing a relationship between a steel sheet surface temperature and a heat flux.

FIG. 4 is a diagram showing a relationship between a steel sheet surface temperature and a water density.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Steel plate 2 Table roller 3 Cooling water header 4 Slit nozzle 5 Spray nozzle 6 Drain roll 7 Flow control valve 8 Cooling nozzle 9 Cooling nozzle 10 Air nozzle

Claims (2)

[Claims] 1. A method for on-line controlled cooling of a hot-rolled high-temperature steel sheet, wherein said controlled cooling is divided into two stages of pre-stage cooling and post-stage cooling, and said pre-stage cooling is performed by whole-surface film-boiling cooling. Wherein the post-stage cooling is performed by nucleate boiling cooling to a predetermined cooling stop temperature. 2. The water volume density of the cooling water in the pre-stage cooling is set in a range of 100 to 300 l / min.m ² , the surface temperature of the steel sheet at the end of the pre-stage cooling is set in a range of 550 to 600 ° C., and a water flow rate of the cooling water at the start of post-cool and 800 l / min.m ² or more, the method of claim 1, wherein.